Apparatus and method for utilization of a high-speed optical wavelength tuning source

ABSTRACT

Exemplary embodiments of apparatus, source arrangement and method for, e.g., providing high-speed wavelength tuning can be provided. According to one exemplary embodiment, at least one arrangement can be provided which is configured to emit an electromagnetic radiation that (i) has a spectrum whose mean frequency changes at an absolute rate that is greater than about 6000 (or 2000) terahertz per millisecond, (ii) whose mean frequency changes over a range that is greater than about 10 terahertz, and/or (iii) has an instantaneous line width that is less than about 15 gigahertz. According to another exemplary embodiment, at least one arrangement can be provided configured to, periodically and as a function of time, select at least one first electro-magnetic radiation based on a mean frequency of the at least one first electro-magnetic radiation, with the periodic selection being performed at a first characteristic period. The mean frequency can vary linearly over time, wherein the apparatus can emit at least one second electromagnetic radiation that has a spectrum whose mean frequency changes periodically as a function of time with a second characteristic period. Further, the first characteristic period can be greater than the second characteristic period.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims the benefit of priority fromU.S. Patent Application Ser. No. 61/149,922, filed on Feb. 4, 2009, theentire disclosure of which is incorporated herein by reference.

FIELD OF THE DISCLOSURE

Exemplary embodiments of the present disclosure relates generally tooptical systems, and more particularly to an optical wavelength filtersystem and method for wavelength tuning and a wavelength-swept laserarrangement and methods which can utilize the optical wavelength filtersystem.

BACKGROUND INFORMATION

Considerable effort has been devoted for developing rapidly and widelytunable wavelength laser sources for optical reflectometry, biomedicalimaging, sensor interrogation, and tests and measurements. A narrow linewidth, wide-range and rapid tuning have been obtained by the use of anintra-cavity narrow band wavelength scanning filter. Mode-hopping-free,single-frequency operation has been provided in an extended-cavitysemiconductor laser by using a diffraction grating filter arrangement.To obtain a single-frequency laser operation and to ensure amode-hop-free tuning, however, a complicated mechanical apparatus andlimit the maximum tuning speed may need to be used, conventionally. Oneof the fastest tuning speeds demonstrated using the conventional systemshas been limited to less than 100 nm/s. In certain applications, such asbiomedical imaging, multiple-longitudinal mode operation, correspondingto an instantaneous line width as large or great than 10 GHz, may besufficient. Such width can provide a ranging depth of a few millimetersin tissues in optical coherence tomography and a micrometer-leveltransverse resolution in spectrally-encoded confocal microscopy.

A line width on the order of about 10 GHz is achievable with the use ofan intra-cavity tuning element (such as, e.g., an acousto-optic filter,Fabry-Perot filter, and galvanometer-driven diffraction grating filter).However, the sweep frequency previously provided has been less thanabout 1 kHz, limited by finite tuning speeds of the filters.Higher-speed tuning with a repetition rate greater than 15 kHz may beneeded for video-rate (e.g., greater than 30 frames/second),high-resolution optical imaging in biomedical applications.

Further, a wavelength-swept laser has been described which can usepolygon scanning filter, and that can provide high-speed wavelengthtuning up to about 10,000 nm/ms. While the high-speed polygon basedwavelength-swept light source facilitates high-speed imaging as fast asabout 200 frames/s, wavelength tuning rate as fast as about 10,000nm/ms, maintaining an instantaneous line-width narrower than 0.15 nm hasalready reached to the limit of the current polygon basedwavelength-swept filter.

Indeed, one of the objects of the exemplary embodiments of the presentdisclosure is to reduce or address the deficiencies and/or limitationsof the prior art procedures and systems described herein above. Forexample, with respect to faster tuning, wide wavelength tuning range andnarrow instantaneous line-width at fast tuning rate, there may be a needfor an exemplary embodiment of wavelength scanning filter arrangementand procedure (e.g., a laser procedure).

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE DISCLOSURE

Exemplary embodiments of the present disclosure relate to apparatus, asource arrangement and method for light-wave filtering that can providea high-speed wavelength-swept light with a broad spectral tuning rangeand a narrow instantaneous line-width. In one exemplary embodiment ofthe present disclosure, the exemplary high-speed wavelength-swept lasercan include a high-finesse wavelength tuning filter which may use apolygon scanning mirror, a short length laser resonator with asemiconductor optical amplifier (SOA) gain medium, a time interleavingoptical delay line, and/or a booster optical amplifier, e.g., at anoutput port of the source. Certain exemplary optical components andexemplary arrangement and a short length laser cavity can facilitate ahigh-speed wavelength sweep over a broad tuning range with a narrowinstantaneous line-width. In one exemplary configuration, wavelengthvariation rates greater than about 41,000 nm/ms with an instantaneousline-width narrower than about 0.2 nm can be obtained. The exemplarylaser resonator can include, e.g., a unidirectional ring, or a linearcavity, with a particularly-designed semiconductor optical gain mediumto maximize the gain and to minimize the cavity length of the lasercavity.

Thus, an exemplary embodiment of an apparatus according to the presentdisclosure can be provided. In this exemplary embodiment, at least onearrangement can be provided which is configured to emit anelectromagnetic radiation. Such exemplary radiation can have a spectrumwhose mean frequency changes (i) at an absolute rate that is greaterthan about 6000 terahertz per millisecond, and (ii) over a range that isgreater than about 10 terahertz.

For example, the mean frequency can change repeatedly at a repetitionrate that is greater than 5 kilohertz. The spectrum can have a tuningrange whose center is approximately centered at 1300 nm. The spectrumcan have an instantaneous line width that is smaller than 100 gigahertz,or even 35 gigahertz. A polygon arrangement can be provided which can beconfigured to receive at least one signal that is associated with theemitted electromagnetic radiation, and reflect and/or deflect thesignal(s) to a further location. In addition, a laser resonating systemcan be provided which can form an optical circuit, and be configured tocontrol a spatial mode of the emitted electromagnetic radiation. Theexemplary apparatus can cause the emitted electromagnetic radiation topropagate substantially unidirectionally within at least one portion ofthe laser resonating system.

Another exemplary embodiment of an apparatus according to the presentdisclosure can be provided. In this exemplary embodiment, at least onearrangement can also be provided which is configured to emit anelectromagnetic radiation. Such exemplary radiation can have (i) aspectrum whose mean frequency changes at an absolute rate that isgreater than about 2000 terahertz per millisecond, and (ii) aninstantaneous line width that is less than about 15 gigahertz.

According to still exemplary embodiment, at least one arrangement can beprovided configured to, periodically and as a function of time, selectat least one first electro-magnetic radiation based on a mean frequencyof the at least one first electro-magnetic radiation, with the periodicselection being performed at a first characteristic period. The meanfrequency can vary linearly over time, wherein the apparatus can emit atleast one second electromagnetic radiation that has a spectrum whosemean frequency changes periodically as a function of time with a secondcharacteristic period. Further, the first characteristic period can begreater than the second characteristic period.

For example, the first characteristic period can be at least two timesthat of the second characteristic period. The second electromagneticradiation(s) can have a spectrum whose mean frequency changes at anabsolute rate that is greater than about 2000 terahertz per millisecond.The second electromagnetic radiation(s) can also have an instantaneousline width that is less than about 15 gigahertz.

According to still another exemplary embodiment of the presentdisclosure, an apparatus can be provided. The exemplary apparatus caninclude at least one arrangement which can be configured to select atleast one first electro-magnetic radiation based on a mean frequency ofthe first electro-magnetic radiation(s). The selection can be performedby the arrangement(s) with a first characteristic free spectral range.For example, the apparatus can emit at least one second electromagneticradiation that can have a spectrum whose mean frequency changesperiodically as a function of time with a second characteristic freespectral range. The first characteristic free spectral range can begreater than the second characteristic free spectral range.

According to one exemplary variant of the present disclosure, the firstcharacteristic period can be at least two times that of the secondcharacteristic period. The second electromagnetic radiation(s) can havea spectrum whose mean frequency changes at an absolute rate that isgreater than about 2000 terahertz per millisecond. The secondelectromagnetic radiation(s) can also have an instantaneous line widththat is less than about 15 gigahertz. The mean frequency can varylinearly over time.

In still a further exemplary embodiment, an apparatus can be provided.This exemplary apparatus can include at least one arrangement which isconfigured to, periodically and as a function of time, select at leastone first electro-magnetic radiation based on a mean frequency of thefirst electro-magnetic radiation(s). The periodic selection can beperformed at a first characteristic period. The exemplary apparatus canalso be configured to emit at least one second electromagnetic radiationthat has a spectrum whose mean frequency changes periodically as afunction of time with a second characteristic period. For example, thefirst characteristic period can be greater than two times the durationof the second characteristic period.

These and other objects, features and advantages of the exemplaryembodiment of the present disclosure will become apparent upon readingthe following detailed description of the exemplary embodiments of thepresent disclosure, when taken in conjunction with the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present disclosure willbecome apparent from the following detailed description taken inconjunction with the accompanying figures showing illustrativeembodiments of the present disclosure, in which:

FIG. 1 is a schematic diagram of a first exemplary embodiment of ahigh-speed wavelength-swept laser system/arrangement with aunidirectional ring resonator according to the present disclosure;

FIG. 2 is a combinational block and functional diagram illustrating anexemplary use of a time interleaving optical delay line and a boosteramplifier, according to an exemplary embodiment of the presentdisclosure;

FIG. 3( a) is a diagram of an exemplary embodiment of a timeinterleaving fiber delay line with a cascaded Mach-Zehnderinterferometer for copying and pasting of the laser output, according tothe present disclosure;

FIG. 3( b) is a diagram of another exemplary embodiment of the timeinterleaving fiber delay line with a tree-like fiber delays with Faradayrotator mirror for copying and pasting the laser output, according tothe present disclosure;

FIG. 4 is a schematic diagram of a second exemplary embodiment of thehigh-speed wavelength-swept laser system/arrangement with aunidirectional ring resonator, according to the present disclosure;

FIG. 5 is a schematic diagram of an third embodiment of the high-speedwavelength-swept laser system/arrangement with a linear cavityresonator, according to the present disclosure; and

FIG. 6 is a schematic diagram of a fourth exemplary embodiment of thehigh-speed wavelength-swept laser system/arrangement with the linearcavity resonator, according to the present disclosure.

Throughout the figures, the same reference numerals and characters,unless otherwise stated, are used to denote like features, elements,components or portions of the illustrated embodiments. Moreover, whilethe subject disclosure will now be described in detail with reference tothe figures, it is done so in connection with the illustrativeembodiments. It is intended that changes and modifications can be madeto the described exemplary embodiments without departing from the truescope and spirit of the subject disclosure as defined by the appendedclaims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 shows a schematic diagram of an exemplary embodiment of ahigh-speed wavelength-swept laser system/arrangement in accordance withthe present disclosure. For example, the exemplary lasersystem/arrangement can comprise an optical high-finesse wavelengthtuning filter 1′ that can utilize a polygon scanning mirror 700, a shortlength unidirectional ring resonator cavity 2′, and a time interleavingoptical delay line 400 with a booster optical amplifier 500. Theexemplary optical wavelength tuning filter 1′ can be configured as areflection-type filter which can have substantially similar or identicalinput and output ports. The exemplary wavelength tuning filter 1′ caninclude a diffraction grating 220, a telescope with a set of lenses 260and 262, a polygon scanning mirror 700, and end reflectors 280 and 282.

The exemplary implementation of the wavelength tuning procedureaccording to an exemplary embodiment of the present disclosure can besimilar to the previous approach [see description of Oh, Optics Letters30(23), 3159-3161 (2005) and disclosure of International PatentPublication WO 2005/001401] including the exemplary procedure of fourtimes reflection on the polygon mirror (e.g., twice×double pass) forlarge finesse of the filter but without a folded telescope according tothe exemplary embodiment of the present disclosure to reduce the opticalpath length. In the conventional approach, a polygon scanning mirrorwith a large number of facets can be used to increase the tuningrepetition rate. Although increasing the number of facet may assist withincreasing the tuning repetition rate, it may not be the most efficientway to increase the wavelength sweep rate with given filter bandwidth,which can be characterized by the total wavelength sweep range per unittime and unit width of the filter pass-band.

For example, with fixed filter bandwidth, the finesse of a polygonscanning filter can be inversely proportional to N²,

${F = {\frac{\left( {F\; S\; R} \right)}{\left( {\delta \; \lambda} \right)_{3\; d\; B}} = {\frac{\pi^{3}}{\sqrt{\ln \; 2}}\frac{D}{\lambda \; N^{2}}}}},$

where (FSR) is the free spectral range and (δλ)_(3dB) is the 3 dBbandwidth of the filter, respectively, D is the diameter of the polygon,and N is the number of facet of the polygon. Therefore, the wavelengthsweep rate can generally become proportional to 1/N. With theconventional polygon based wavelength-swept laser with N=128 providing115 kHz tuning repetition rate (9,200 nm/ms), it is possible to obtainabout 41,800 nm/ms wavelength sweep rate, e.g., if a polygon mirror 700is used with N=28. According to the exemplary embodiment of the presentdisclosure, the tuning repetition rate can be about 25 kHz can be used.However, the FSR (which determines the wavelength tuning range) canbecome about 1,664 nm.

When the gain in the laser cavity supports lasing over ˜104 nmbandwidth, a continuous wavelength sweep over about 104 nm can beobtained with about 6.25% duty cycle 322. An idle portion of theexemplary laser output can then be completely filled up by making 15copies of the laser output, properly delaying, and pasting with the timeinterleaving optical delay line 400, resulting in, e.g., about 16wavelength sweeps in a single facet-to-facet rotation period of thepolygon 700. As a result, the repetition rate of about 400 kHz (25kHz×16) can be obtained. The output of the delay line 400 can beamplified using the booster-amplifier 500.

When wavelength sweep rate is high, a reduction of an optical pathlength of the laser cavity can become important. For example, in orderto support long photon intracavity lifetime by reducing the filtercenter wavelength shift per cavity round trip, the length of the laserresonator can be reduced and/or minimized by using a short focal lengthlenses for the telescope (e.g., lenses) 260, 262. In the ring cavity 2′,an isolator, which facilitates a unidirectional lasing, can beintegrated inside the semiconductor optical amplifier (SOA) gain medium100, and fiber pigtails 120, 122 of the SOA 100 can be re-connetorizedat ends/connectors 140, 142, thus leaving a minimum length of theoptical fiber. In order to form a ring oscillation, the output of thepolygon filter can be vertically offset and directed to the inputconnector 142 of the SOA 100 by the reflector 200. Additional componentsof the exemplary embodiment shown in FIG. 1 and the exemplary operationthereof shall be described herein below.

FIG. 2 shows a combinational block and functional diagram of anexemplary embodiment according to the present disclosure whichillustrates an exemplary use of a time interleaving optical delay lineand a booster amplifier. For example, the laser output 246 can beprovided directly from the polygon filter 820, for example, that canhave about 25% duty cycle in this exemplary case. Three copies of thisoutput can be directed and made at 244, properly delayed, and thenpasted 840 through the time interleaving delay line 400. Laser outputpower 600 can then be recovered 860 by a booster amplification 500.

FIGS. 3( a) and 3(b) show exemplary embodiments of the time interleavingoptical delay line with optical fibers for copying and pasting of thelaser output according to the present disclosure. To make three copies,for example, three different lengths of fiber delays, 442, 444, and 446can be utilized. These exemplary delayed copies can be pasted to theoriginal laser output 246 by using, e.g., a cascaded Mach-Zehnderinterferometer as shown in the exemplary embodiment of FIG. 3( a) (whichalso illustrates the use of splitter 420, 422, 424 and the outputprovided on line 402). It is also possible to utilize a tree-likeinterferometer with a circulator 403, splitter 421, 423, 425 and Faradayrotator mirrors 460, 462, 464, and 466, so as to reduce or eliminate abirefringence effect in the delay line, as shown in the exemplaryembodiment of FIG. 3( b).

FIG. 4 shows a schematic diagram of a second exemplary embodiment of thehigh-speed wavelength-swept laser system/arrangement according to thepresent disclosure. Using this exemplary embodiment, the output of thelaser cavity can be obtained after the SOA (e.g., gain medium) 100, andthrough the free-space output beam splitter 240. Since a filtered light320 can be amplified by the SOA 100 before being coupled at point 300,the output light from the cavity 302 can contain, e.g., a particularamount of a amplitude spontaneous emission (ASE) during an idle portionof the output. To prevent or reduce such ASE from being overlapped withthe copied laser output in the delay line/arrangement 400, the SOA gainmedium 100 can be modulated to be switched off at point 102 during theidle period of the laser resonator output.

FIG. 5 shows a schematic diagram of a third exemplary embodiment of thehigh-speed wavelength-swept laser system/arrangement with an exemplarylinear cavity configuration according to the present disclosure. Forexample, a SOA gain medium 110 having a mirror coating 122 can beprovided on one end of the system/arrangement which can support a linearcavity oscillation of the laser arrangement. The output from theoscillator of the laser can be coupled by the Free-space optic beamsplitter 240.

FIG. 6 shows a schematic diagram of a fourth exemplary embodiment of thehigh-speed wavelength-swept laser system/arrangement having linearcavity configuration according to the present disclosure. In thisexemplary embodiment, the SOA gain medium 112 having a partialreflection coating 124 can be provided on one end of the exemplarysystem/arrangement which can support a linear cavity oscillation of thelaser and the transmitted light through the partial reflector 124. SuchSOA gain medium 112 can be coupled as a laser output, and the directedlight can be directed 244 to the optical delay line 400. The ASE in theidle portion of the laser resonator output can be reduced and/oreliminated by the gain switching 102 of the SOA 112.

The foregoing merely illustrates the principles of the invention.Various modifications and alterations to the described embodiments willbe apparent to those skilled in the art in view of the teachings herein.Indeed, the arrangements, systems and methods according to the exemplaryembodiments of the present disclosure can be used with and/or implementany OCT system, OFDI system, SD-OCT system or other imaging systems, andfor example with those described in International Patent ApplicationPCT/US2004/029148, filed Sep. 8, 2004 which published as InternationalPatent Publication No. WO 2005/047813 on May 26, 2005, U.S. patentapplication Ser. No. 11/266,779, filed Nov. 2, 2005 which published asU.S. Patent Publication No. 2006/0093276 on May 4, 2006, and U.S. patentapplication Ser. No. 10/501,276, filed Jul. 9, 2004 which published asU.S. Patent Publication No. 2005/0018201 on Jan. 27, 2005, and U.S.Patent Publication No. 2002/0122246, published on May 9, 2002, thedisclosures of which are incorporated by reference herein in theirentireties. It will thus be appreciated that those skilled in the artwill be able to devise numerous systems, arrangements and methods which,although not explicitly shown or described herein, embody the principlesof the invention and are thus within the spirit and scope of the presentdisclosure. In addition, to the extent that the prior art knowledge hasnot been explicitly incorporated by reference herein above, it isexplicitly being incorporated herein in its entirety. All publicationsreferenced herein above are incorporated herein by reference in theirentireties.

1. An apparatus comprising: at least one arrangement configured to emitan electromagnetic radiation that has a spectrum whose mean frequencychanges (i) at an absolute rate that is greater than about 6000terahertz per millisecond, and (ii) over a range that is greater thanabout 10 terahertz.
 2. The apparatus according to claim 1, wherein themean frequency changes repeatedly at a repetition rate that is greaterthan 5 kilohertz.
 3. The apparatus according to claim 2, wherein thespectrum has a tuning range whose center is approximately centered at1300 nm.
 4. The apparatus according to claim 2, wherein the spectrum hasan instantaneous line width that is smaller than 100 gigahertz.
 5. Theapparatus according to claim 2, wherein the spectrum has aninstantaneous line width that is smaller than 35 gigahertz.
 6. Theapparatus according to claim 1, further comprising a polygon arrangementwhich is configured to receive at least one signal that is associatedwith the emitted electromagnetic radiation, and at least one of reflector deflect the at least one signal to a further location.
 7. Theapparatus according to claim 1, further comprising a laser resonatingsystem forming an optical circuit, and configured to control a spatialmode of the emitted electromagnetic radiation.
 8. The apparatusaccording to claim 7, wherein the apparatus causes the emittedelectromagnetic radiation to propagate substantially unidirectionallywithin at least one portion of the laser resonating system.
 9. Anapparatus comprising: an arrangement configured to emit anelectromagnetic radiation that has (i) a spectrum whose mean frequencychanges at an absolute rate that is greater than about 2000 terahertzper millisecond, and (ii) an instantaneous line width that is less thanabout 15 gigahertz.
 10. The apparatus according to claim 9, furthercomprising a polygon arrangement which is configured to receive at leastone signal that is associated with the emitted electromagneticradiation, and at least one of reflect or deflect the at least onesignal to a further location.
 11. The apparatus according to claim 9,further comprising a laser resonating system forming an optical circuit,and configured to control a spatial mode of the emitted electromagneticradiation.
 12. The apparatus according to claim 11, wherein theapparatus causes the emitted electromagnetic radiation to propagatesubstantially unidirectionally within at least one portion of the laserresonating system.
 13. An apparatus comprising: at least one arrangementconfigured to, periodically and as a function of time, select at leastone first electro-magnetic radiation based on a mean frequency of the atleast one first electro-magnetic radiation, the periodic selection beingperformed at a first characteristic period, wherein the mean frequencyvaries linearly over time, wherein the apparatus is configured to emitat least one second electromagnetic radiation that has a spectrum whosemean frequency changes periodically as a function of time with a secondcharacteristic period, and wherein the first characteristic period isgreater than the second characteristic period.
 14. The apparatusaccording to claim 13, wherein the first characteristic period is atleast two times that of the second characteristic period.
 15. Theapparatus according to claim 13, wherein the at least one secondelectromagnetic radiation has a spectrum whose mean frequency changes atan absolute rate that is greater than about 2000 terahertz permillisecond.
 16. The apparatus according to claim 15, wherein the atleast one second electromagnetic radiation has an instantaneous linewidth that is less than about 15 gigahertz.
 17. An apparatus comprising:at least one arrangement configured to select at least one firstelectro-magnetic radiation based on a mean frequency of the at least onefirst electro-magnetic radiation, the selection being performed by theat least one arrangement with a first characteristic free spectralrange, wherein the apparatus emits at least one second electromagneticradiation that has a spectrum whose mean frequency changes periodicallyas a function of time with a second characteristic free spectral range,and wherein the first characteristic free spectral range is greater thanthe second characteristic free spectral range.
 18. The apparatusaccording to claim 17, wherein the first characteristic period is atleast two times that of the second characteristic period.
 19. Theapparatus according to claim 17, wherein the at least one secondelectromagnetic radiation has a spectrum whose mean frequency changes atan absolute rate that is greater than about 2000 terahertz permillisecond.
 20. The apparatus according to claim 19, wherein the atleast one second electromagnetic radiation has an instantaneous linewidth that is less than about 15 gigahertz.
 21. The apparatus accordingto claim 17, wherein the mean frequency varies linearly over time. 22.An apparatus comprising: at least one arrangement configured to,periodically and as a function of time, select at least one firstelectro-magnetic radiation based on a mean frequency of the at least onefirst electro-magnetic radiation, the periodic selection being performedat a first characteristic period, wherein the apparatus is configured toemit at least one second electromagnetic radiation that has a spectrumwhose mean frequency changes periodically as a function of time with asecond characteristic period, and wherein the first characteristicperiod is greater than two times the duration of the secondcharacteristic period.